In tape transport apparatus wherein a recording tape is fed between a supply reel and a takeup reel, it has been found necessary to determine when the end of a tape has been totally disengaged from the reel supplying the tape to the other reel. At such point in time, the motors that are driving the tape reels and the tape need to be braked and inactivated. If the motors continue to rotate after the end of the tape has been removed from the reel that is supplying the tape, then the tape and particularly the end portion of the tape will be subject to physical damage with possible loss of recorded data, among other things. This problem appears in tape transports wherein the tape is completely unwound from one reel and wound onto the other reel, and in tape apparatus utilizing closed video-cassettes in which the tape is captive and the ends are not released from one tape reel to the other. In the latter case, if the end of the tape is not sensed, there is the danger that the tape movement, which is usually at high speed, would not be properly stopped and that the tape end would disengage from the reel.
In the past, an end of tape sensor constituted a conductive element attached to the end of the tape which triggered an electrical signal to indicate that the tape had reached its physical end and thus had been completely released from the reel on which it had been wound. This was implemented by attaching a material different than that of the recording tape, which required additional labor and had its shortcomings in that it was subject to separation from the tape with use. Another approach was to record a signal at the end of the tape which could be read out by an associated sensing circuit. This arrangement also added labor and expense as well as the requirement for space allocation on the tape.
More recent approaches to end of tape sensing employ light sensing devices, such as photodiodes, which are activated by a light source that provides radiation to the photodiode once the end of the tape has moved past the optical path. However, it was found that light sensing devices are sensitive to any stray light, which may come from ambient light or strobe light, by way of example. To overcome this problem, relatively large mechanical shields were provided to surround the sensing element or photodiode so that the sensor did not receive the spurious light. With the trend towards making data processing apparatus, such as tape transports, more compact, less expensive and more reliable, these mechanical shields posed significant problems of space, cost and reliability. Therefore, it would be advantageous to provide an end of tape sensor that is compact and inexpensive, and which selectively responds to a specified light source.